High-tensile steel plates are used for automobiles to reduce their weight. A high-tensile steel plate has an increased strength compared with ordinary steel plates. It is strengthened by adding alloy components, controlling a structure or others. Although definition of a high-tensile steel plate differs depending on the country and the manufacturer, the one having a strength of approximately 490 MPa or higher is referred to as a high-tensile steel. By using a high-tensile steel plate, it is possible to ensure sufficient strength with a thin or small steel plate member. A high-tensile steel plate is poor in formability and workability because of its high strength. Therefore, it is difficult to ensure high accuracy in forming or in subsequent working. In addition, the durability of a die used in forming or subsequent working tends to deteriorate.
In light of these problems, an ordinary steel plate softer than a high-tensile steel plate is formed or worked and then conducting a heating and quenching treatment. Also, a steel plate heated by a heat of the heating and quenching treatment is formed or worked. The latter working method is called a hot-press working. According to these methods, it is possible to satisfy both excellent formability, workability and strength.
Partial strengthening of a steel plate member by the heating and quenching treatment is classified roughly into a method of partially hardening a part of a steel plate to strengthen only the part, and a method of partially preventing hardening of a part of a steel plate by suppressing temperature rise in the part though the entire steel plate is heated. In the present description, a part that is partially hardened in the steel plate is called a strengthened part, and a part that is not hardened is called a non-strengthened part. Such uneven strengthening of a steel plate member aims at preventing an ununiform distribution of stress generated by external force, facilitating appropriate deformation for absorbing or letting out the external force, and obtaining a soft part for post workings such as piercing or trimming. In case of partial hardening, induction hardening, according to which a strengthened part is easily designated, is utilized (Patent Document 1). In case of partially preventing hardening, a method of suppressing temperature rise of a non-strengthened part is utilized (Patent Document 2 to Patent Document 4).
Patent Document 2 discloses a method of conducting hardening by applying current between electrodes connected to a steel plate (hereinafter, referred to as direct energization heating). In this method, a non-strengthened part is not hardened though the steel plate is entirely heated (1) by contacting a block having higher conductivity than the steel plate and partly diverting the current to the block to prevent a temperature rise of the non-strengthened part (FIG. 1 to FIG. 3 in Patent Document 2); (2) by spraying a cooling gas to suppress temperature rise in the non-strengthened part (FIG. 4 to FIG. 6 in Patent Document 2), or (3) by contacting a ceramics block and others having lower conductivity than the steel plate and absorbing the heat of the non-strengthened part to suppress temperature rise (FIG. 7 to FIG. 8 in Patent Document 2).
Patent Document 3 discloses a hardening method too in which a non-strengthened part is not hardened though the steel plate is entirely heated. In this method, a non-strengthened part of a steel plate is sandwiched by a heat insulating materials. The steel plate and the heat insulating materials are put into an electric furnace. Then, by heating the non-strengthened part to less than a transformation end temperature (AC3) and heating the remaining strengthened part to the transformation end temperature (AC3) or higher, the steel plate is partially strengthened ([claim 1] of Patent Document 3). The steel plate taken out of the electric furnace is subjected to a press working (hot press working) for cooling, or further subjected to a post working ([claim 3] of Patent Document 3). Rock wool, glass wool, ceramic fiber, and heat resistant brick are exemplified as heat insulating materials (Patent Document 3, [0026]).
In patent Document 4, a temperature control member is brought into contact with a non-strengthened part of a steel plate in the course of heating the steel plate in order to control the non-strengthened part to a transformation start temperature (AC1) or less. The temperature control member is formed of a nonconductive material, and is controlled to the same temperature as the steel plate under heating ([claim 3], [claim 5], [0029] of Patent Document 4). As a heating method, direct energization heating is exemplified ([claim 4] and others of Patent Document 4). While the method of Patent Document 4 is stated as a solution for the problem of the method of Patent Document 3, it can also be regarded as improvement of the above (3) disclosed in Patent Document 2.